The Coyote Hills – a Relic of the Age of Dinosaurs
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AUTUMN 2016 Volume 39, Number 3 The Coyote Hills – A Relic of the Age of Dinosaurs By Bruce Rogers, sediment covering of the older ocean floor US Geological Survey (retired) called the Farallon Plate, was scraped off as The Coyote Hills are located at the it was subducted (pushed under) the ad- eastern end of the Dumbarton Bridge in vancing North American Plate, then heated, the City of Fremont in southern Alameda squeezed, and generally mistreated. County. The bulk of the hills are north of Rather quickly, geologically speak- ing, some of these rocks that made up the Highway 84, where a regional park is named Farallon Plate top-most layer, were thrust after the hills. The smallest of the hills, under the North American Plate, and then located south of Highway 84, is where the pushed back up to the surface. Further plate San Francisco Bay National Wildlife Refuge tectonic movements cracked, bent, and Headquarters resides. moved these large blocks of rock around the They form an irregular, linear ridge western edge of North America. Some were roughly parallel to the western Coast Ranges smashed against the then coast where they and eastern Diablo Range. Surrounded by were welded into place about 70 million a piedmont of grassy fields to the east and years ago. mixed marshland and salt evaporator ponds Along the joint between these two to the west, they look strangely out of place. plates, exotic rocks formed nearly 10 miles No other natural hills in the east bay exist beneath the ocean were then “squeezed” in such close proximity to the San Francisco up along the deeply reaching boundary Bay. Their rugged red, brown, and greenish faults and intruded into the already highly Figure 1. Radiolarian Chert by the Harrier rocks also look unlike most of the rocks mish-mash of assorted rocks at coast side. Spur Trail at Don Edwards Refuge. forming the surrounding hills and moun- Subsequently, more tectonic movements Photo: Jose Garcia tains. These seemingly out of place and pushed up the Coast Ranges, similar to exotic rocks comprising the Coyote Hills, pushing up folds in a rug. These rocks have its convoluted migration to Coyote Hills, it however, offer a glimpse into the far distant fascinated geologist for nearly 150 years. was metamorphosed, squeezed, heated, and past of Central California. First mapped in San Francisco in the late subjected to alkali fluids nearly 500 degrees To start, we must travel back to the 1800s, they were named the Franciscan Fahrenheit in temperature. Once welded to dawn of the Age of Dinosaurs, nearly 200 Assemblage or Group after the San the coast, this pervasively fractured rock was million years ago. The Fremont and Newark Francisco-founding Franciscans. deeply weathered to an ocherous brownish- area was then covered by the proto-Pacific Before we go further into how the yellow color. Thin veins of milky quartz Ocean. Underwater volcanoes poured black Coyote Hills became a “high tide” island and white calcite deposited from hot water basalt lava out onto the ocean bed floor (one surrounded by sea water only at high solutions encountered during its wild ride while masses of silica ooze slowly, accumu- tide), we should take a quick look at the commonly pattern these rocks. lating in the adjacent, deeper ocean basins. rocks themselves. The most common rock Nearly as plentiful as the greenstone, All these rocks were originally deposited in exposed in the hills is a dark greenish-black bright reddish-orange chert (Figure 1) tropical waters just north of the equator and colored rock called greenstone. makes up most of the higher hills and pin- perhaps several thousand miles southwest of Greenstone, denoted as “gs” on the geo- nacled outcrops of the Coyote Hills. Chert their present location. logic map (Figure 2), is basalt lava, common started its life as silica accumulating on the A great deal of time passed as the in such places as Hawaii. Originally poured floors of deep ocean basins. Literally billions North American Plate pushed its way west, out onto the ocean floor as pillow-shaped overriding the older ocean floor. The thin blobs (amazingly called pillow basalt) during continued next page from page 1 of tiny silica-shelled animals called radiolar- ians lived, died, and sank to form a jelly-like ooze. Compressed for millennia, this ooze slowly turned into hard, brittle chert. Traces of iron and manganese give the chert its red, yellow, orange, blackish-blue, and green coloration. In many places, thin beds of manganese oxides remain between the chert layers as thin, glossy bluish-black coatings. Less common is feldspar-rich sand- stone called graywacke. In the Franciscan Assemblage, graywacke usually forms the bulk of the rocks, but here it is far less common. Most of these rocks are deeply weathered. One can also see graywacke by digging nearly 30 feet down to fresh rock where the original light gray color can be seen. Very minor amounts of highly meta- morphosed limestone, denoted as “Is” on the geologic map, deposited on the original offshore island chain have been found in the hills. The scraps of this white to rose-colored rock only occur in a few places in the Coyote Hills including the northwestern end and center of the range in Coyote Hills Regional Park. Limestone often contains tiny fossils used to date the rocks and to figure out their origin. This limestone, in contrast to the more common reef deposits, was formed atop of submerged mountains called seamounts. Hot water Figure 2. Geologic map of the coyote hills. from the cooling basalt precipitated lime This package of former deep sea rocks of magnesium-rich solutions, sprinkling from the seawater to form the limestone. are now considered one of 11 series of “ter- thumb-sized bobbles of the orange to Relatively pure limestone is colored creamy ranes” or groups of similar rock groups in tan-colored mineral magnesite into the white while those colored in various shades the San Francisco Bay Area and surrounding serpentine. of red and pink have more iron impurities Central California geographic area. The Once the Coyote Hills attained their within it. greenstone-chert-graywacke combination of final resting place, the forces of water and Let us return to the days of yesteryear old ocean bottom rocks, is called the Marin wind began their patient work of reducing and see just what became of the hodgepodge Headlands terrane and extends from Coyote them to low slopes adjacent to the San of rocks of slightly differing age and mode Hills north under San Francisco Bay, up Francisco Bay. This, however, would take of origin. About 24 million years ago, through central San Francisco, and on into some time. the east-over-west motion of the North the Marin Headlands where it was first San Francisco Bay formed from a grassy American Plate slightly diverged and a large mapped by geologists nearly 120 years ago. valley about 12,000 years ago as slowly splinter of the coastal apron (rock) was split Sometime during all this shuffling of rising seawater from the melting of glaciers off. The resulting fracture separating this large plates of rock, deep crust rocks made covered the aprons surrounding the hill. large block from the rest of North America of dark green minerals were slowly extruded Saltwater marshes formed along the edges is now called the San Andreas fault system upwards along the nearly vertical faults as a of the hills and extended outward nearly a (Figure 3). The Hayward and Calaveras plastic sheet. Partly metamorphosed by hot mile into the bay. Slowly the adjacent slopes faults are part of this system. A host of water solutions in the crust during their epic of the Diablo Range were eroded down to small, shorter faults are also present, one journey, the rocks were changed into bluish- provide great quantities of sandy and silty being the Silver Creek fault that extends green serpentine, California’s State Rock. sediment that Alameda Creek spread to from the northeastern San Jose area north This mass was intruded into the Marin nearly engulf the eastern shore of the Coyote towards Hayward. This fault actually Headlands terrane, as well as others, along Hills. As a result, the hills were formerly marks the eastern edge of the Coyote Hills. the major faults separating the terranes “high tide” islands, having a seawater moat Because these hills that are so distinct from from each other. Indeed, wherever you find during high tides. the surrounding rocks and structure, it is serpentine, you are looking at a profoundly The first peoples to occupy the area postulated that a similar fault marks the deep fault millions of years old. Waning arrived about 10,000 years ago and, finding western edge of the hills. heated water solutions added a final pulse the climate and plentiful resources to their Page 2 liking, began to exploit these resources. Chert was used to make tools and weapons. Grinding pits for processing nuts and seeds were worn into both chert boulders and greenstone outcrops. Modern settlers followed their prede- cessors and diked off marshlands to form salt evaporation ponds, creating the largest natural salt distillery in North America. In adjacent, drained marshes to the east of Coyote Hills, cattle ranches and dairy and vegetable farms thrived during the 1800s and early 1900s before everyone found the delightful climate of the Bay Area and moved in. Modern developers, too, found the flanks of the Coyote Hills attractive and opened up several quarries to mine crushed rock aggregate for concrete. Indeed, the former Dumbarton Quarry once extended nearly 120 feet below sea level in its final Figure 3.